There has been growing interest in the use of antibody drug conjugates (ADCs) for the treatment of cancer as mounting data suggests an increase in anti-tumor effectiveness and reduced toxicity, compared with the administration of unlabeled antibodies in combination with chemotherapy.1-4 The two ADCs that are currently approved by the FDA, brentuximab vedotin (Adcetris) and ado-trastuzumab emtansine (T-DM1; Kadcyla) are both heterogeneous mixtures in terms of both the location of the drug and the number of drugs per IgG. Recent evidence has shown that differentially labeled antibodies, i.e. labeled at different locations and with different numbers, can have distinct therapeutic and pharmacokinetic properties5 and some subpopulations can show little, if any, therapeutic activity yet account for most of the toxicity.6-8 Therefore, there has been a movement towards the development of site-specific ADCs, which are precisely labeled with drugs at pre-defined locations. Currently, there are four general approaches for the generation of site-specific ADCs. These include the introduction of cysteine-tags into the genetic code of IgG,6, 9 the use of unnatural amino acids,10 the addition of peptide tags that are recognized and modified with enzymes (e.g. formylglycine generating enzyme or transglutaminase),8, 11 and glycan modifications (e.g. via glycotransferase).12 While each approach has its own unique advantages, shortcomings can include low product yields, inefficient drug conjugation, unstable and/or hydrophobic antibody-drug linker chemistry, or incompatibility with glycosylated antibodies. We propose to further develop a new site-specific bioconjugation approach, Proximity-Based Sortase Ligation (PBSL), that can produce ADCs in high yields, allows different drugs to be site-specifically added to the heavy and/or light chains, is compatible with glycosylated IgG, and offers unlimited flexibility in antibody-drug linker chemistry. Therefore, we believe that this technology will provide a new, favorable approach for the production of site- specific ADCs that will be of interest to the pharmaceutical industry. The specific aims for this proposal are: Aim 1: Use PBSL to produce site-specific anti-prostate specific membrane antigen (PSMA)-MMAE ADCs and characterize their properties; Aim 2: Evaluate the binding and efficacy of site-specific anti-PSMA-MMAE ADCs in vitro